Problem: Which of these numbers is prime? ${5,\ 10,\ 75,\ 85,\ 91}$
A prime number has exactly two factors: 1 and itself. The factors of 5 are 1 and 5. The factors of 10 are 1, 2, 5, and 10. The factors of 75 are 1, 3, 5, 15, 25, and 75. The factors of 85 are 1, 5, 17, and 85. The factors of 91 are 1, 7, 13, and 91. Thus, 5 is a prime number.